FIELD OF THE INVENTION
The present invention concerns the digital transmission of data and in particular data transmitted by the multiplexing method known as SDH (Synchronous Digital Hierarchy).
There are two separate justification techniques at the Virtual Container 12 (VC12) level within Synchronous Digital Hierarchy (SDH). They are both used at the same time within SDH, but not in the initial justification process at the entry point to the SDH network.
By initially using both techniques, Complementary Justification eliminates the possibility of a large amount of wander being introduced by an SDH network.
Reducing the amount of network wander enables the size of any boundary buffer to be reduced.
The fitting of boundary buffers on the exit from the SDH network can greatly reduce the relative wander.
Multiplexing techniques enable a multiplicity of data streams often called tributaries to be carried by a single transmission system. When a tributary data stream has a clock rate which is independent of the clock rate of the transmission system then justification techniques are required in order to maintain the integrity of the tributary data stream. When a transmission system is demultiplexed the individual tributary clocks have to be recreated in order to recreate the tributary data streams. The accuracy with which the tributary clocks are recreated is dependent on the initial justification and the final dejustification/desynchronization techniques employed as well as any intermediate network rejustification techniques used within the intermediate transmission network.
The poor performance of the justification techniques specified by the SDH recommendations compared with the earlier justification techniques employed by the Plesiochronous Digital Hierarchy resulted in U.S. Pat. No. 5,172,376 imported herein by reference. However although the technique described performs very well it has not been included in the SDH recommendations. To implement that technique requires the intermediate network rejustifiers within a network as well as the initial justification functions and final dejustification/desynchronisation functions to be modified. The overall performance of the present invention although inferior to the above mentioned patent has the advantage of only requiring the modification of the initial justification function for the basic implementation and both the initial justification and the final dejustification/desynchronisation function for the double-ended implementation.
The poor performance of the justification techniques specified by the SDH recommendations is caused by several factors including: the use of positive/zero/negative justification; the use of byte justification; the use of large amounts of hysteresis; and the use of irregular spaced multiplex formats. The problem of irregular spaced multiplexed formats can be overcome by the use of certain internal design techniques. The problems caused by the use of large amounts of hysteresis with positive/zero/negative byte for all intermediate network rejustification functions cannot be overcome if the normal operating methods as described by the SDH recommendations are followed.
The use of positive/zero/negative justification is intended to minimize the number of justification actions. Unfortunately this also minimizes the amount of phase control information carried by the transmission system with a consequential degradation of the overall transfer characteristics.
The use of either byte justification or bit justification for the initial justification is permitted by the SDH recommendations for tributaries although not for the high bandwidth AU (Administration Unit) payloads. When the initial rejustification used is bit justification and there are intermediate network byte rejustification functions then the dejustification/desynchronization function has to be able to accept both bit and byte justifications.
For SDH the justification actions can be infrequent occurrences because tributary clocks and the transmission clock are all multiples of 8 kHz although not necessarily the same 8 kHz source.